Previous electronic distance meters have been for use with stationary targets. Such distance meters provide information of the form "the distance to the target is so much." Since the target is stationary, the distance to the target does not change and it is not necessary to indicate when the target is (or was) at the indicated distance.
A distance meter that measures distances to moving targets must provide information of the form "at such and such a time the distance to the target was (or is) so much". Unambiguous distance information is provided by issuing a time mark signal when the target is at the indicated distance. Delays in the internal processing of the measurement must be accounted for in the timing of the time mark signal.
Ever present delays produced by filtering the position sensing signals give rise to two types of error. First, the phase delay, or apparent propagation delay, through the filter produces a time offset between when the target was at a given distance and when a phase measurement of the filtered signals indicates the given distance. Furthermore, the doppler effect causes the amount of the offset to be a function of target velocity. Second, in the presence of doppler shifts in frequency the filter produces differential phase shifts in the signals whose phases are to be compared. This produces an error in the indicated distance. If the error in the distance is converted to the time the target required to travel the amount of the error, and is then added to the apparent propagation delay, the resulting sum is a constant provided the velocity of the target does not exceed certain limits. By incorporating the offset into the timing of the time mark signal the distance meter provides accurate distance/time mark pairs.
A distance meter that responds to moving targets can also be used to determine target velocity if the time interval between measurements is supplied along with the distance information. The measured time intervals can be accumulated to form a time axis for the successive distance measurements. However, if the distance meter cannot promptly initiate a measurement in synchronism with an external event, there will be some offset between the accumulated time axis and the external event. While the offset can be measured and later removed, that is an awkward maneuver compared to removing the offset from the accumulated time axis in the first place, by providing an initial time interval whose time equals the duration between the external event and the next completed distance measurement. Such an initial time interval can be obtained by providing a means to asynchronously reset the internal time interval timer.
Distance meters that are suitable for measuring distances to moving targets may be relied upon to provide essential data in control systems. Damage to the system or injury to personnel may result if the distance meter should be unable to obtain valid measurements. To facilitate decision making by the control system, the distance meter provides coded information regarding the integrity of the target signal path and the apparent nature of the inability to obtain a measurement.
Accordingly, a primary object of the present invention is to provide a means of supplying a time mark signal that correctly corresponds with distance data obtained by a measurement to a moving target.
A second object of the invention is to provide means to coordinate time interval measurements for successive distance measurements with an initial external event.
Another object of the invention is to supply coded information relating to the integrity of the target signal path and the general nature of any inability to obtain a distance measurement.